- Moving get_canonical_filename() from a public function to a method in handler.
- Adding a helper method is_canonical_filename() to handler.
- Adding helper methods left(), substr(), starts_with() to Lex_cstring.
- Adding helper methods is_sane(), buffer_overlaps(),
  max_data_size() to CharBuffer.
- Adding append_casedn() to CharBuffer. It implements the main functionality
  that replaces the being removed my_casedn_str() call.
- Adding a class Table_path_buffer,
  a descendant of CharBuffer with size FN_REFLEN.
- Changing get_canonical_filename() to get a pointer to Table_path_buffer
  instead just a pointer to char.
- Changing the data type of the "path" parameter and the return type of
  get_canonical_filename() from char* to Lex_cstring.